Hello Pete,
AE5PL Lists schreef:
> #1 - ALOHA is not transmitting in-the-blind. It is a much more complex
> protocol which is significantly different from APRS. The term has been
> misused extensively here to justify people using the statistics gathered
> in that project which are not related, in any way, to VHF APRS
Sorry but we are talking about the channel access characteristics only,
not the full ALOHA network. ALOHA in this context is "pure ALOHA" which
was the first type of channel access that the University of Hawaii used in
1972. This type of channel access has a maximum troughtput at 18.4%
channel utilisation. With less packets the channel is under-utilized and
with more packets throughput drops due to many collisions.
So this is independent of any retries. For the complete ALOHA network the
retries by the bighter layes caused the network collapse as soon as you
went over the 18.4% channel utilisation. From 0% to 18.4% the channel
thoughput increases and the channel is stable. Above 18.4% the channel
becomes unstable since more packets are lost, more need to be retried and
the load just increases so fast that the network comes to a standstill.
So behaviour of the higher layers causes the ALOHA network to breakdown
once you go "over the top". The transmitters have to back-off. This is
very different from APRS, APRS runs happely when the channel is 40% loaded
- only you cannot get nearly as much data as with 18.4% load but it
doesn't get worse like the first ALOHA network would.
The U of H made serveral improvements to pure ALOHA to enhance maximum
channel capacity. Two improvements were to use fixed length packets and to
introduce timeslots. When we talk about ALOHA channel access in
conjunction with APRS it always refers to "pure ALOHA".
For the term ALOHA for this type of channel access, I don't know a better
term that is widely known and accepted. If you would stay in line with the
terminology like CSMA, CSMACD, TDMA, FDMA, CDMA, then pure ALOHA should be
called "MA". But that's not the way it is in literature...
Now for your APRS area being mostly CDMA. I have to agree for
home-stations, digipeaters and others with good antenna's and terrain that
allows you to hear eachother. But if I take my TH-D7 outside with its
rubber-duck, the TH-D7 can only hear the digi - at least frequently...
You have to be aware that all these small devices like the TH-D7 with
rubber-duck will actually wait until the big stations in the area (which
they all can hear) stop transmitting and all these low power/small
footprint devices are competing for the same periods of silence when the
big stations shut up. So CSMA increases the probability that these small
devices are jamming eachother - causing both packets to be corrupted
instead of one making is to the digi.
While reading your mail I also had another strange feeling. If every
station can already hear all its neigbours, why do you have a digipeater?
That only doubles the ammount packets for no good reason. Otherwise your
90% must be flawed since every digipeater can theoretically transmit at
most 50% of the time since it needs to receive the packets first. If you
hear multiple digis then you have definatly too many in too close proximity.
Home stations, digipeaters and stations with a good antenna and/or high
power should never transmit blind. But I realy do not see much of a
problem with low power/low footprint devices.
Okay, I'll leave it with that. I think the raised points are clear and I
even think we don't disagree much. Avoid however to take things for
granted because you once learned that doing something is a bad thing. Keep
an open mind and dare to question these kind of general assumptions.
By the way, did you know DAMA also works without carrier detection? In
DAMA the master (the node) does the listening for you, when the slave (a
connected station) is polled it has to respond immediately, regardless of
what's going on on the channel. The idea is that the master controls who
transmits and when, so the slave just obeys without question. The master
is responible for polling the slave when the channel is clear, the slave
is not allowed to add any extra delay in responding; when the response is
not received immediately the master will poll the next slave. There cannot
be any uncontrolled stations on the DAMA channel, except for sending SABM
for connection-setup.
Another example of running without carrier detection is Full Duplex
connection on a point to point link. Its just assumed that you are the
only transmitting station on one of the two QRG's of the FD link. All our
packet interlinks on 23 cm work that way.
Kind regards,
Henk.